The Science of Sleep

Linda Geddes explores the secret life of your sleeping body.

Why do we sleep and what happens when we sleep? Your eyelids may be closed, but your mind and body are hard at work while you sleep. Far from being a dispensable luxury, getting a good night’s rest could hold the secret to a longer, happier life. 

By the time you reach seventy, you will have spent approximately 20-23 years of your life asleep. To some, this period of inactivity amounts to a profound waste of time. “Don’t sleep any more than you have to”, advises Donald Trump, who claims to sleep for just three to four hours each night.  

Among sleep researchers though, there’s a growing consensus that far from being cognitive dead-time, all those years spent asleep will have laid the foundations of the person you’ve become. Sleep is fundamental to our ability to learn and consolidate memories. It influences our interactions with other people, and our performance at work. For sportspeople, good sleep can spell the difference between Olympic gold and professional obsolescence. If you could bottle sleep and sell it, the on-pack label might read, ‘Warning: this product will make you slimmer, happier, more productive, smarter, healthier, better looking, better company, and extend your life’. 

So, what precisely is going on when we drift into this daily period of unconsciousness? And how does sleep produce all these benefits?

“Warning: this product will make you slimmer, happier, more productive, smarter, healthier, better looking, better company, and extend your life.” 

What is sleep?

Most life forms show daily 24-hour patterns of activity and rest, which could arguably represent some form of slumber. Certainly, all mammals sleep, and if they’re deprived of it for any length of time, they’ll compensate by spending a longer time asleep when they finally get the opportunity to do so.

This pressure to sleep is thought to arise because of the build-up of chemicals such as adenosine in the brain during wakefulness. You could think of it as sand falling through an hourglass, measuring the amount of time you’ve spent awake; once enough of it has accumulated the urge to sleep becomes irresistible, forcing most of us to bed. When we sleep, the hourglass is flipped and the whole process begins again. 

Girls looking sleepy on bed

The other factor influencing when and how long we sleep for is our internal circadian rhythm or body clock. This creates windows of sleepiness and alertness over the 24-hour period and explains both why we sleep for a prolonged and consolidated chunk at night, and why we often find it so difficult to sleep during the day. An exception to this is the so-called ‘post-lunch dip’ when the signals from our circadian alerting system struggle to keep up with the mounting sleep pressure in our brains. If you’ve slept poorly the previous night, this may be a good time to take a brief nap or siesta – as people in many countries do. 

Without these circadian signals, regular catnapping would be our normal sleep pattern, because sleep pressure builds- and dissipates relatively quickly. Instead, these two systems combine forces, meaning we stay awake and relatively alert for around 16 hours each day, and then sleep in a consolidated chunk each night.  Describing it as a chunk isn’t quite correct, though, because human sleep is broken into 90-minute cycles, and each of these can be further subdivided into periods of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep.  

hypnogram graph of sleep cycles

The first stage we enter when we fall asleep is NREM stage 1 sleep (dozing), from which we rapidly move into NREM stage 2 (light sleep). This is the state in which we spend the greatest proportion of our sleep, and during which our conscious experience of the world evaporates.

Next, we move into NREM stages 3 and 4 (deep sleep), from which it is very difficult to be roused. If you get woken up during deep sleep, you’re likely to feel extremely groggy. Deep sleep is when we release growth hormone, which enables us to repair and rejuvenate our tissues. It is also when phenomena like night terrors, bedwetting, sleepwalking and sleeptalking occur.

Finally, we enter REM sleep, when our bodies become temporarily paralyzed. Although dreaming can occur during both REM and NREM sleep our dreams tend to be longer and more vivid during REM sleep. The proportion of NREM and REM sleep we get during each 90-minute sleep cycle varies through the night. During the first few cycles, NREM sleep predominates, whereas during later cycles we spend longer in REM sleep.

3 reasons why we sleep: to repair and replenish our cells and tissues; to conserve energy; and to consolidate learning and memories

Why do we do it? 

There are three broad theories as to why we sleep: to repair and replenish our cells and tissues; to conserve energy; and to consolidate learning and memories. Sleeping when it’s dark, when our visual systems are less able to detect predators, may also have improved our survival prospects. Possibly sleep has evolved to achieve all these functions, although different activities appear to be associated with the various sleep stages.   

Drawing of the cells of the cerebellum by Santiago Ramón y Cajal

Memory is a good example of this. A key function of NREM sleep appears to be the weeding out of unnecessary connections between brain cells, while REM sleep helps to strengthen those connections. In his book Why We Sleep, the neuroscientist Matthew Walker likens the interplay between REM and NREM sleep to moulding a sculpture out of clay. During the first couple of sleep cycles, NREM sleep excavates and removes large amounts of superfluous material – a mass of old and new memories the brain no longer wishes to keep – while shorter bursts of REM sleep smooth and mould the sculpture’s basic shape. During later cycles REM sleep further strengthens and defines these features, with just a small amount of input from NREM sleep. 

All these types of sleep are important for your memory. Deep NREM sleep is associated with the consolidation of newly acquired memories, so if you’re studying for an exam, it is this type of sleep that will help those facts to stick. Short and intense bursts of light NREM sleep (called spindles), which punctuate REM sleep during later cycles, appear to be important for helping to transfer recently acquired memories to longer term storage; while REM sleep helps cross-reference these newly acquired memories with the back catalogue of older stored memories. This is where creative insights and abstract connections often spring from and may be one reason why sleeping on a problem often provides a solution.

Another function of REM sleep is in helping to fine-tune our emotions. If we’re deprived of it, e.g. by going to bed late and setting an alarm clock to wake up early, we become less adept at regulating our emotions or reading those of other people, which has the potential to impact our relationships.

Another function of REM sleep is helping to fine-tune our emotions.

Sleep and health

Sleep also fulfils a vital housekeeping function.  Researchers recently discovered a system which pumps cerebrospinal fluid through the brain while we sleep, flushing out waste – such as toxic beta-amyloid protein that accumulates during the day and which is associated with the development of Alzheimer’s disease. 

When we don’t get enough sleep we want to eat more.

Sleep provides a period of rest and recuperation for our digestive systems. Up to a tenth of the cells lining our gut are damaged each day through the simple act of consuming and processing food and need to be repaired or replaced at night.  It helps us to regulate the hormones which control our appetite. When we don’t get enough sleep, we secrete less of a hormone called leptin which helps us to feel full after eating, and more of the ‘hunger-hormone’ ghrelin, so we want to eat more. We’re also more likely to succumb to these urges because sleep deprivation affects parts of the brain responsible for impulse control. The good news is that getting a full night’s rest restores this system, so sleeping well could help you to maintain a healthy body weight.  

Woman meditating on cliff top facing the sea
Photo by Patrick Malleret on Unsplash

The list of health benefits associated with sleep continues. When we sleep, we release substances that tweak our immune systems and our blood pressure; sleep deprivation is associated with a poorer immune response and an increased risk of cardiovascular disease. In part, this is because of its effects on our sympathetic nervous system, which directs our responses to stressful or dangerous situations.  When we don’t get enough sleep, our fight-or-flight response is triggered, resulting in an elevated heart rate and blood pressure, which can damage our hearts and blood vessels if it is sustained for long periods of time. Deep NREM sleep, on the other hand, lowers our pulse and blood pressure, giving our cardiovascular systems a chance to rest and recuperate. Chronic sleep deprivation is also associated with cancer, in part because an overactive fight-or-flight response sets up a state of sustained inflammation in our bodies, and cancers use inflammation to help them grow and spread.

How much sleep? 

So, sleep clearly fulfils an important purpose, but how much of it do we really need? Although eight hours is often quoted as the ideal, this isn’t necessarily supported by studies of tribes living in Africa or South America, which might provide some insight into our evolutionary past.  

For instance, a study which looked at sleep timing within the Hadza people of Tanzania, the San of Namibia and the Tsimane of Bolivia found that they spent between 6.9 and 8.5 hours in bed each night, although the actual amount of time they spent asleep was 5.7 to 7.2 hours. Most people averaged an extra hour of sleep in winter – although they took more naps in summer.  

“Individuals who routinely sleep less than six hours risk
compromising their health and well-being.”

US National Sleep Foundation

On the other hand, the US National Sleep Foundation (NSF) advises that adults aged 18 to 64 need to get seven to nine hours of sleep per night (seven to eight hours for those aged over 65), and that individuals who routinely sleep less than six hours (five hours for those over 65) risk compromising their health and well-being. The NSF reached this conclusion after asking 18 leading sleep researchers to review more than 300 scientific publications. Children and teenagers need even more sleep, they say. 

Possibly there are outliers who can get by on less sleep than this. For instance, researchers recently identified several genetic mutations which enable individuals to feel fully rested on as little as 4.5 hours of sleep per night. They have also genetically engineered mice to carry these mutations and found that they sleep for around 55 minutes less than normal mice. The mutations affect an area of the brain which helps to regulate sleep, called the dorsal pons. 

Pouring milk into an expresso
Photo by Tyler Nix on Unsplash

To identify whether you’re getting enough sleep, the NSF advises asking yourself whether you are productive, healthy and happy on seven hours of sleep, or whether you need more to get into high gear? Also, do you depend on caffeine to get through your day, or feel sleepy when driving? These may be indications  that you need more sleep than you’re currently getting.

These are all signs that your sleep quality may need to improve:
regularly wake up more than once a night, spend less than 85% of your time in bed asleep, or take more than 30 minutes to fall asleep.

The NSF also emphasises the importance of sleep quality. If you regularly wake up more than once a night and it takes more than 20 minutes for you to fall back to sleep; spend less than 85% of your time in bed asleep; or take more than 30 minutes to fall asleep in the first place, these are all signs that your sleep quality may need to improve. Both cutting short your sleep, and frequently waking up during the night can impact the amount of NREM and REM sleep you get and leave you feeling groggy the next day, and less capable of managing your emotions. Longer term, it may increase your risk of Alzheimer’s disease, so it is worth approaching  your doctor for advice, the NSF suggests.

Sleep and performance

Even if you feel fine, there may yet be benefits to trying to get more sleep. One study in American college basketball players found that when they were instructed to get as much extra sleep as possible by spending at least ten hours in bed each night, they recorded faster sprint times and greater shooting accuracy. A follow-up study found that after just five days of such ‘sleep-loading’, professional baseball players showed a 13 % improvement in their reaction speeds and they responded 66 milliseconds faster on a visual attention test which required them to identify a specific object or feature among similar-looking objects or features – even though the actual amount of sleep they got only increased from 6.3 to 6.9 hours each night.  

Boosting sleep is also likely to increase your performance at work. Even taking a brief nap has been shown to boost mental alertness and task performance.  Studies in teenagers, meanwhile, have suggested that shifting their school start times later to allow them more sleep, results in them feeling more academically engaged and happier. People’s circadian clocks naturally shift later during adolescence, which means they may struggle to fall asleep at night, and therefore cut short their sleep. When one British school changed its start time from 8.50am to 10am, rates of absence due to illness fell dramatically, while the proportion of pupils gaining ‘good’ GCSEs increased from 34% to 53% two years after the change.

Silhouette of a boy reading at sunset.
Photo by Aaron Burden on Unsplash

Sleep is integral to our physical and mental well-being, so it is worth investing in it. The old mentality of ‘I’ll sleep when I’m dead’ is outdated, and indeed dangerous. Without it, we rob our minds and bodies of the ability to reset and rejuvenate. Sleep enables us to live a full and healthy life. We sleep to live.     

Linda Geddes is the author of Chasing The Sun: The new science of sunlight and how it shapes our bodies and minds. 


Why We Sleep: The new science of sleep and dreams. Matthew Walker, 2017, Scribner.

Sleep: The myth of 8 hours, the power of naps…and the new plan to recharge your body and mind. Nick Littlehales, 2016, Penguin Random House

Sleep: A very short introduction. Steven W. Lockley and Russell G. Foster. Oxford University Press 2012.

Chasing The Sun: The new science of sunlight and how it shapes our minds and bodies. Linda Geddes, 2019, Wellcome Collection. 

The Drive to Sleep and Our Internal Clock – Healthy Sleep

Gut microbiome diversity is associated with sleep physiology in humans – Plos One

Natural sleep and its seasonal variations in three pre-industrial societies
– Current Biology

A second mutation that makes people need less sleep has been found – New Scientist

What is good quality sleep? – National Sleep Foundation

5 signs that your sleep quality is poor (and how to fix it) – National Sleep Foundation

Sleep extension improves response time, reduces fatigue in professional baseball players – Science Daily

The Effects of Sleep Extension on the Athletic Performance of Collegiate Basketball Players – Sleep Research Society

Later school start times study – American Academy of Sleep Medicine

A 10.00 a.m. school start time improves health and performance of students aged 13-16 – Frontiers in Human Neuroscience
Caffeine Effects on Sleep Taken 0, 3, or 6 Hours before going to bed – Journal of Clinical Sleep Medicine

Sleep trackers can prompt sleep problems – Science Daily

sleep to live

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